Teacher-student interaction in classrooms is perceived to be crucial for learning. Previous research has compared the interaction in classrooms where a second language is used as the medium of instruction (MoI) with those where the mother tongue is used. This has been done mainly via qualitative impressions. The current study adopted a mixed method design to examine objectively and comprehensively the classroom interaction in a sample of Hong Kong secondary schools that used Chinese as the MoI from Grade 7 to 9, but then switched to English from Grade 10 onwards (i.e. MoI-switching schools). By doing so, this study aimed to identify the possible effect of a change in the MoI on classroom interaction and its potential for learning. Sixty lessons were observed in Grade 9 and 10 in three MoI-switching schools and compared to two schools where English was used throughout all the grades. The results show that, when the MoI changed from students' first to second language, lessons tended to become more teacher-centred and there were fewer opportunities for negotiation of meaning and scaffolding. These findings have significant implications for the relationship between the MoI and classroom interaction and provide important insights into pedagogy and teacher development.[Copyright of International Journal of Bilingual Education and Bilingualism is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/13670050.2011.588307]
